People who will claim with their dying breath “I can’t lose weight.” or “I can’t gain weight.” can be shown to do so when their caloric intake and caloric output is strictly controlled (meaning in a metabolic ward where every meal is meticulously weighed and measured) to accomplish one or the other. Create a large enough caloric deficit, or a large enough caloric surplus, and something simply HAS to happen. Either metabolism adapts (see below) or bodyweight changes.
It might not be fun, it might not be sustainable, but it will happen. As a buddy of mine once asked: “Why don’t you ever see a fat person come out of a concentration camp?” But that’s essentially what a fat person claiming they can’t lose weight on 500 calories per day is suggesting can happen. Because in the face of low enough calories and sufficient activity, weight has to be lost. Or the person dies. Nothing else can happen. Yet folks seem intent on believing that somehow the basic laws of the universe apply to everyone but them.
It’s not uncommon to find individuals who will claim that “I don’t eat that much and I gain weight” or “I eat a ton and can’t gain weight.” which seems to put me in my place and prove me wrong. In research, there’s typically been two attitudes towards these types of statements. The first is that there is truly some metabolic/thermodynamic miracle occurring. The second is that people are just really bad at estimating their caloric intake and expenditure. Turns out that number two is what’s usually going on.
Invariably, when you get an honest assessment of the person’s food intake (just accept that it can be done right now), their estimates are way off from reality. Studies show that people may under (or overestimate) their true caloric intake by up to 50%. Basically, unless they’ve done it for a while, most people are simply horrible at estimating how much food they are actually eating. Same thing for exercise, people tend to vastly overestimate how many calories they’ve burned.
So when you ask them to compare their food intake to their energy expenditure, they’ll tend to say that they eat very little and burn very much, and be totally off of reality. So what they are expecting to happen to their weight isn’t the same as what’s going to happen to their weight (based on the realities of the energy balance equation).
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This is why, although it’s a huge pain in the ass (at least initially), meticulously tracking food intake for a few days (and by this I mean getting a food scale and measuring cups/spoons) can be exceedingly informative (or depressing depending on how you look at it). When people who swear up and down that they “Just don’t eat that much” sit down and track it, they invariably find that they are eating two to three times as much as they though. Without fail.
Anyhow, and putting it rather bluntly: if there were truly an exception to this simple thermodynamic rule, the government would need to study it because that person would be a living breathing fusion reactor, able to make calories out of thin air ; or able to burn them off to an unlimited degree.
The research, however, is very clear: not everybody has it as easy as some folks do. Some people’s bodies are, in fact, demonstrably more resistant to weight loss (or gain) than others. Not that they can’t lose (or gain) weight but it comes off or on more slowly. More accurately, their bodies fight back harder.
Researchers call these folks Diet Resistant and the reasons behind this resistance is just starting to be determined. It probably has to do with how these individuals brains perceive changes in caloric intake which determines how their brains react to those changes. Some people’s bodies simply increase metabolic rate more quickly (or drop it more quickly) in response to increased or decreased calories. You can see similar variations in terms of what’s lost during dieting; given the same diet and exercise program, some people will lose a lot more muscle than another.
And we all have that one friend who eats nothing but ice cream and soda and never gains a pound. Of course, when you look closely, you find that the person really isn’t eating as much as it looks like overall, or they are only eating that one big meal per day that you happened to see, or they are burning it off because they are constantly moving (in essence, they fidget the excess calories off), or they compensate the next day after eating a lot and eat very little so that overall they maintain their weight.
These people’s brains sense the caloric excess more readily and either blunt hunger harder and faster, or get the person to move more, to burn it off. The same thing happens in reverse, some people’s metabolic rates slow down faster when calories are restricted, or makes them move around less during the day so they burn fewer calories, making further fat loss a lot harder.
So there is no doubt that there are individual differences and efficiencies between people, that probably explains why you can find one person who reports near-magical results with nearly every diet out there: they happened to hit the one that just ‘fit’ their individual metabolism and chemistry. It would be silly to ignore all of that and I do hate being silly.
But that doesn’t change the fundamental rules of thermodynamics which apply to everybody and everything. Given 100 calories, the most you can store is 100 calories. Sure, one person may only store 75, while another stores all 100, but 100 is still the maximum. It’s a physiological impossibility to store more than you actually ate because you can’t make something out of nothing. There’s lots of things like this, that you simply can’t do. You can’t make gold out of lead, you can’t find an honest politician, and you can’t store 500 calories if you only ate 300.
So when a 300 pound individual, who probably has a maintenance intake of 4000+ calories, says that they gained weight on 1400 calories I have to be very leery of how true that is. Either they are that 1 in 100,000 person with a metabolic rate below 1400 at that bodyweight (who has never been found to exist in any study on the topic over a span of about 5+ decades), or they aren’t being accurate in how much food they are eating or how many calories they are burning each day. You can probably guess which one I think it is. And, so we’re clear, I’m not saying that they are deliberately lying, either, I want to make that very clear. They are just as bad as everybody else at estimating their caloric intake and expenditure. Which is apparently pretty bad.
Which is why you can’t magically gain weight on 1000 calories per day if your maintenance intake is 2000 calories per day. Either your body will mobilize stored fuels, or it will slow down metabolic rate to 1000 to put you back into balance (and no study has ever shown the latter to occur in the absence of rather massive weight loss). Something has to happen. But weight gain on sub-maintenance calories isn’t one of them.
It’s also why you can’t not gain weight on 3000 calories per day if your metabolic rate is only 2000 calories per day. Either you start storing fuel or your body is speeding up metabolic rate to compensate. Something has to happen.
Summing up:
You.
Are.
Not.
Different.